A SOLID STATE SENSOR FOR CARBON MONOXIDE

Abstract: A molybdenum, tungsten or vanadium salt-palladium, ruthenium or osmium salt composition for CO detection is made reversible by addition of ferric, chromium (VI) or cerium (IV) ion. The system is made more CO specific by adding an interference control salt which forms white or colorless precipitates with interfering species. The operational and shelf life are extended by a mixture of counterions which prevent reduction of the catalyst; the acetate counterion is particularly useful as a redox property modifier. In a solid state optical sensor for CO, the chemistry is contained in a polymer embedding matrix, with permeation enhancer, if required. Solubility of the chemistry in the polymer matrix is enhanced by introducing lipophilic counterions. The matrix with embedded sensing chemistry is coated on an optical support to form an optical transducer which may be an interference type transducer.

Description: A Solid State Sensor for Carbon Monoxide BACKGROUND OF THE INVENTION This invention relates generally to chemical sensors for toxic gases, and in particular to carbon monoxide sensors. Sensors and sensing systems for detecting toxic pollutants are gaining increasing prominence in process control, residential environment, transportation vehicles, and in work places. Carbon monoxide is an odorless poisonous gas, with an exposure limit of only 35 ppm. Solid state sensing devices impart numerous advantages. In particular, solid state sensors are user friendly, possess extended shelf life and operational life, can be easily mass produced, and reduce the risk of improper handling by the user. The prior art for detecting carbon monoxide colori etrically involves the use of palladium and molybdenum compounds, as described by M. Shepherd, Anal. Chem. JL9 (2), 77, (1947). The use of these compounds have also been reported as early as 1910 by C. Zenghelis, Z. Anal. Chem., 40. 429, (1910) and the literature was reviewed in 1935 by J. Schmidt, "Das Kohlenoxyd" , Akad Verlag, Leipzig, P 186, (1935) . This chemistry also appears in "Spot Tests in Inorganic Analysis" by F. Feigel, V. Anger, R. Oesper, Elsevier Publishing Company, New York, P. 168 (1972) . U.S. Patent 3,112,999 to Grosskopf is directed to a solid state carbon dioxide sensing device. The basic chemical reactions for the palladium catalyzed reduction of molybdenum by carbon monoxide are as follows: Mo +6 + CO > Mo + + C0 2 (1) Pd +2 + CO > Pd" + C0 2 (2) Pd" + Mo +0 > Pd +2 + Mo +3 (3) The reaction of carbon monoxide with molybdenum (Equation 1) is very sluggish. Therefore, palladium is employed as a catalyst, where palladium is first reduced by carbon monoxide. Reduced palladium, Pd", in turn then reduces the molybdenum to lower oxidation states, the most common one being Mo +3 , which is also known as "molybdenum blue". Thus, a slightly yellow solution is changed to a blue color. The intensity of the blue color directly correlates to the extent of CO exposure. Unfortunately, however, the reduced molybdenum is rather stable, and does not quickly go back to the initial oxidation state so that the same chemistry could be recycled. This irreversibility makes this chemistry of limited use. To make the system reversible, therefore, there must be a secondary reaction which converts Mo +3 back to Mo +ή , i.e., an oxidizer must be present. A successful reversing agent must meet the following two important criteria: (a) the regeneration reaction should be fast, and (b) the regenerating agent itself should quickly revert back to the starting state for the next cycle. A reversible CO sensor is shown by Shuler et al, U.S. Patent 4,043,934 which has a Mo, W or V color forming agent, Pd catalyst and Cu, Ni or Fe reversing agent. The sensing reagent is deposited on an inert carrier which is hydrophilic or contains water or OH " groups, e.g. silica gel, alumina, polymeric alcohol, polyglycol, cellulose, glass wool and sponges. M.K. Goldstein in U.S. Patent 5,063,164 describes a biomimetic sensor for detecting the presence of airborne toxins including CO. That patent suggests several recipes for making regenerable sensors, but does not address the criteria or requirements for a successful reversible sensor; nor does it address the chemistry or mechanisms to make the CO sensor completely specific. Goldstein shows a solid state CO sensor having five components: (1) palladium salt, (2) molybdenum and /or tungsten salt or acid salt, (3) copper salt, (4) cyclodextrin molecular encapsulant which encapsulates at least one but not all of the other components, and (5) chloride salt, all impregnated into a porous substrate. The Mo, /Pd/Cu system is as in Shuler. The improvement is the encapsulant which extends sensor lifetime. An excess of chloride ions are also provided to extend lifetime. The substrates include silica- gel beads and porous glass, in which diffusion of gases can be rather slow. Goldstein's patent does not reveal (a) how fast the reverse reaction occurs, or (b) whether it can stand a drastic environment like 100 % CO. U.S. Patent No. 5,063,164 uses Cu + salts as the reversing agent. Cu ++ and Cu + ions are very stable at ambient atmospheric conditions. Therefore, the Cu ++ /Cu + pair does not fully meet the criteria of a successful reversing agent for a deadly toxic gas like carbon monoxide. SUMMARY OF THE INVENTION According to one aspect of the invention, there is provided a CO sensor comprising, a CO sensing chemistry comprising: a color forming agent which is reduced by CO and produces a color change; a catalyst which is reduced by CO and which thereby also reduces the color forming agent; a reversing agent which oxidizes the reduced color forming agent back to its initial state; a redox property modifying agent which prevents reduction of the catalyst and the color forming agent in the absence of CO. According to another aspect of the invention, there is provided a method for increasing the solubility of an inorganic salt having an ion and associated counterion in an organic matrix comprising exchanging the associated counterion with a lipophilic counterion and embedding the ion-lipophilic counterion pair in the organic matrix. Accordingly, the invention may provide an improved optical sensor for carbon monoxide, and /or a liquid state or solid state CO sensor. Preferably, the invention provides a CO sensor which is reversible, eliminates interferences, and has an extended lifetime. The invention is a CO sensor which preferably includes a color forming agent, a catalyst, and a reversing agent. The CO sensor can further include an interference suppressing agent, and a redox property modifier. The sensor can be formed of an aqueous solution or in solid state, with an improved embedding matrix, a gas permeation agent, and nonporous optical substrates. The preferred matrix is a polymer matrix. Solubility of inorganic salts in the polymer may be improved by lipophilic counterion exchange. The invention is preferably a reversible CO sensor formed of an aqueous solution of (1) molybdenum, tungsten or vanadium salts or acid salts, (2) palladium, ruthenium or osmium salt, (3) iron, chromium or cerium salt. The palladium, ruthenium or osmium salt may provide palladium (II) , ruthenium (VIII) or osmium (VIII) ion as a catalyst. The iron, chromium or cerium salt may provide ferric ion, chromium (VI) ion or cerium (IV) ion as a reversing agent. The solution further can include an interference suppressing agent which forms a white or colorless precipitate to eliminate interferences and increase specificity. The invention further may include a long-life CO sensor based on palladium or other catalyst with mixed counterions which act as a redox property modifier for the catalyst. The CO sensor may be formed of an aqueous solution of (1) molybdenum, tungsten or vanadium salts or acid salts, (2) palladium, ruthenium or osmium salt, (3) Pd(II) or other catalyst redox controlling counterion producing salt, (4) Fe +3 , Cr +0 or Ce +4 ion as the reversing agent, and (5) sodium salts or other interference suppressing agents. The CO sensor can also be formed in the solid state by embedding the CO sensing chemistry in a solid state matrix. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. la is a diagram of a Pd(II) ion surrounded by water dipoles. Fig. lb is a diagram of a Pd(II) ion surrounded by sulfate and acetate counterions. Fig. 2 is a schematic view of a solid state CO sensor measurement system. Figs. 2A,B,C illustrate ATR and absorption solid state CO sensors. Fig. 3 is the absorption spectrum of the CO sensing chemistry. Fig. 4 shows the recovery time of the CO sensing chemistry. Fig. 5 is a schematic view of an interferometric measurement CO sensor. Figs. 6A-D illustrate various ions with lipophilic counterions, DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The CO sensing reagent according to the invention includes: A. Color forming agent: Molybdenum ion is preferred; tungsten or vanadium can also be used. B. Catalyst: Palladium is preferred; ruthenium and osmium are new catalysts according to the invention. C. Reversing agent: Ferric ion is preferred and copper or nickel can also be used; chromium and cerium are new reversing agents according to the invention. D. Redox property modifier: Acetic acid salts are preferred. E. Interference suppressing agent: sodium ion is preferred for hydrogen sulfide interference. Thus, the invention adds new agents, for redox property modification and/or interference suppression, to combinations of known indicators, catalysts and reversing agents. The invention also provides new catalysts and reversing agents. The preferred color forming agent is molybdenum as described herein. However, tungsten and vanadium are also known indicators for CO. The preferred catalyst is palladium, Pd(II) , as described herein. However, in accordance with the invention, ruthenium, Ru(VIII) , and osmium, Os(VIII) , can also be used for the catalyzed reduction of molybdenum by CO. The choice of a reversing agent depends on its ability to produce the desired reactions only, and nothing more. The first requisite is that the Mo +3 formed with the reaction of Pd +2 , CO and Mo +6 goes back to Mo +0 and no other valence states of the Mo. This requires a reversing compound whose redox potential closely matches that of Mo +3 going to Mo +6 so that the reaction proceeds spontaneously in the thermodynamic sense. The second requirement is that the selected oxidizing agent, when used, regenerates itself and not a series of compounds of various chemical formula and valence states. Based on these criteria, ferric ion is preferred (irrespective of the counter ion) as the reversing agent in the formulation of a CO sensing chemistry. The reverse reaction proceeds as follows: Mo +3 + 3Fe +3 > Mo +0 + 3Fe +2 (4) Eventually, by air oxidation Fe +2 returns to Fe +3 to be available for reuse. Alternatively, chromium (VI) ion or cerium (IV) ion can be used as the reversing agent, or other reversing agents including copper or nickel. The amount of ferric ion (such as ferric chloride) , chromium (VI) ion or cerium (IV) ion used strictly depends on the dynamic range of concentrations of CO to be detected and the desired time delay for reversibility. In fact, if time is not a criterion, the reliance on oxygen in the air to cause the reversibility is perfectly acceptable. The CO sensor chemistry is thus a solution of (1) molybdenum, tungsten or vanadium salts or acid salts which provides the Mo +0 , W + ° or V +s ion, (2) palladium, ruthenium or osmium salt which provides the Pd +2 , Ru +S or Os +8 ion, and (3) iron (ferric) , chromium or cerium salt which provides the Fe +3 , Cr +6 or Ce +4 ion. The solution is typically aqueous, but other solvents might be used. The molybdenum acid/salt may be selected from molybdosilicic acid and salts thereof, molybdenum trioxide, heteropolyacids of molybdenum, ammonium molybdate, ammonium molybdophosphate, molybdophosphoric acid, organomolybdenum compounds, and alkali metal or alkaline earth metal salts of the molybdate anion. The tungsten acid/salt may be selected from tungstosilicic acid and salts thereof, tungsten trioxide, tungstophosphoric acid, organotungsten compounds, heteropolyacids of tungsten, ammonium tungstate, and alkali metal or alkaline earth metal salts of the tungstate ion. The vanadium salt may be selected from vanadium (V) oxide, vanadyl phthalocyanine, vanadium (V) trichloride oxide, vanadium (V) trifluoride oxide, vanadium triisopropoxy oxide, vanadyl octaethylporphine. The palladium salt may be selected from palladium sulfate, palladium sulfite, palladium pyrosulfite, palladium chloride, palladium bromide, palladium iodide, palladium perchlorate, palladium acetate, palladium oxalate, palladium citrate, palladium acetylacetonate, allylpalladium bromide, CaPdCf 4 , Na 2 PdCf 4 , K 2 PdC- 4 . The ruthenium and osmium salts include ruthenium (VIII) oxide and osmium (VIII) oxide. The iron (ferric) salt may be selected from ferric chloride, ferric sulfate, ferric bromide, ferric iodide, ferric perchlorate, ferric fluoride, ferric acetylacetonate, ferric ammonium citrate, ferric ammonium sulfate, ferric nitrate, ferric oxalate, ferric phosphate, ammonium ferric citrate, ammonium ferric oxalate. Chromium (VI) salts include but are not limited to potassium dichromate, ammonium dichromate, sodium dichromate, sodium chromate, potassium chromate. Cerium (IV) salts include but are not limited to cerium sulfate, ammonium cerium nitrate, ammonium cerium sulfate. All the salts must be soluble. In operation, the Mo +ft is reduced to Mo +3 by the CO in the presence of the Pd +2 , Ru +K or Os +l< catalyst. The Fe +3 , Cr +6 or Ce +4 then oxidizes the Mo +3 back to Mo +ή . The +0 or V +<i color forming agent behaves similarly. For the detection of CO between 0 and 500 ppm, a typical chemical system consists of: Palladium Sulfate (0.04 wt. %) Molybdosilicic Acid (0.2 wt. %) Ferric Chloride (0.04 wt. %) in aqueous solution. This composition is modified for other dynamic ranges. The second part of the requirement for a CO sensor is that it be specific. All existent CO sensors including the one defined in U.S. Patent 5,063,164 suffer from a variety of interferences of which hydrogen sulfide is most common. For example, palladium sulfide, molybdenum sulfide and copper sulfide (which would be formed in U.S. Patent 5,063,164) are all black/brown which prohibits measurement of the yellow to blue color change when Mo +ft is reduced to Mo +1 . Iron sulfide, which is yellow/green can also cause some problems. The solution, therefore, is to add a fourth component to the system which not only preferentially forms a sulfide, but a white or colorless one which will not interfere with the CO measurement. To accomplish this, sodium chloride (2 wt. %) is incorporated into the system. The sodium ion is the interference suppressing agent. The four (4) component chemical system was extensively tested for interferences with a UV/VIS spectrophotometer. The following table shows the results of these tests. This formulation for a CO sensor indicates no response to the key Occupational Safety and Health Administration (OSHA) interferences. Other salts which are sources of suitable ions can also be used to eliminate interferences and enhance specificity by producing white or colorless precipitates with the interfering species. These salts may be selected from salts of sodium, ammonium, lithium, potassium, calcium, magnesium, beryllium, aluminum, platinum, cobalt, with counterions nitrate, acetate, chloride, sulfate, phosphate, chlorate, nitrite, perchlorate, carbonate, bicarbonate. The interference suppressing ion must preferentially form a precipitate with the interfering species instead of one of the components of the system forming a precipitate with the interfering species. The precipitate could also have a color if the color does not overlap the measurement wavelength or has a resolvable overlap. Extending the shelf life and operational life of the CO sensor is very important. Doing so is difficult, and involves modification of the redox properties of the catalyst. This was achieved in the following way: The catalyst is Palladium(II) ; the following principles also apply to Ru(VIII) , Os(VIII) or other catalysts. When common salts of palladium (PdS0 4 , PdCf 2 , etc.) are dissolved in water, Pd(II) exists in the solvated form, as shown in Fig. 1A. Water molecules form dipoles ( δ ' -δ + ) which surround the Pd(II) ion. The sulfate counterion is also present. The counterions are also similarly solvated. In the aquo form, Pd(II) is energetically very prone to reduction. When Pd +2 is surrounded by neutral molecules like water, ethanol, ethylene glycol, glycerol, and molecules having reducing functional groups, the operational and shelf life of the sensor chemistry are diminished drastically because of reduction without CO. Similar results occur with counterions like S0 4 "2 , Cf " , HP0 4 "2 , etc. Because the species surrounding the Pd +2 determines its redox properties, the properties of Pd +2 can be controlled by carefully selecting its counterions, i.e., by including a redox property modifier. When neat CO is passed through a solution of PdS0 4 in pure water, the solution turns black immediately. However, when PdS0 4 is dissolved in sodium acetate saturated water, it takes fifteen times longer to get dark. The acetate ions (CH-,C0 2 ) now surround the Pd(II) ion as shown in Fig. IB. The sulfate counterion is also present. Thus, the presence of acetate counterion greatly influences the redox properties of Pd +2 . This difference between sulfate and acetate counterions is used to extend lifetime. The invention includes a CO sensing chemistry based on palladium with mixed counterions. By carefully adjusting the proportion of acetate and sulfate, or acetate and chloride counterions, the redox properties of Pd +2 are modulated, thereby prolonging the shelf life and operational life of the sensor. Thus, the invention includes the addition of suitable counterions to the molybdenum/palladium combination. Ferric ion is added for reversibility and sodium salt for specificity as previously described. However, the bicounterion concept is applied to the palladium catalyst to achieve prolonged shelf life and operational life. The redox property modifying counterion can be provided by the following: sodium acetate, potassium acetate, ammonium acetate, magnesium acetate, copper acetate, lithium acetate or other acetic acid salt. Other salts which prevent the reduction of the catalyst and subsequent color forming reaction in the absence of CO could also be used. The amount of counterion to be added is typically in the range of ten times the molar concentration of palladium. The invention in an alternate embodiment is a solid state optical sensor for CO having a sensing material which includes a molybdenum, tungsten or vanadium color forming agent; a palladium, ruthenium or osmium catalyst; and an iron, chromium or cerium reversing agent. A redox property modifier and/or an interference suppressing agent may also be included. The chemistry is contained in a polymer embedding matrix, with permeation enhancer, if required. Solubility of inorganic ions in the polymer is increased by counterion exchange. The matrix with embedded sensing chemistry is coated on an optical substrate to form an optical transducer. The color forming agent produces a measurable color change in the presence of CO. The catalyst speeds up the reaction. The reversing agent converts the color forming agent back to its original state for reuse. The redox property modifier extends the lifetime. The interference suppressing agent removes interfering species. The lipophilic counterion increases solubility of the sensing chemistry in the polymer embedding matrix. As shown in Fig. 2, a solid state CO sensor 10 is placed in a measuring environment. A light signal 12 from source 14 is input into sensor 10. Source 14 is powered by supply 16 and is controlled by feedback circuit 18, if required. Sensor 10 is an optical transducer whose output signal 20 varies as a function of CO exposure, for a given input signal 12. Output signal 20 is detected by detector 22, whose output is connected through amplifier 24 to microprocessor 26 which is connected to output means 28. The sensor can be a hybrid device, or it can be an integrated optic chemical sensor. An incandescent lamp, laser, laser diode or light emitting diode will be employed as the source, while a photodiode, CCD or an interferometer will be employed at the detection end. The device would be used for detecting the instantaneous level of a toxic pollutant as well as for detecting a cumulative amount for a predetermined period of time. The device will make both kinetic and equilibrium measurements. The general working principle of the solid state CO sensor is based on the attenuation of a transmitted light beam during its interaction with the sensing material at a time when the sensing material is exposed to the analyte (CO) . The attenuation of light depends on the concentration of the analyte, as well as on the exposure time. This attenuation can occur because of (a) attenuated total internal reflection (ATR) (Fig. 2A) , (b) absorption (Fig. 2B) , or (c) both. As shown in Fig. 2A, ATR sensor 30 has a sensor coating 32 formed on a portion of optical fiber core 34. Sensor coating 32 includes the CO sensitive sensing chemistry in a suitable CO permeable solid state matrix. An input light beam 35 travels down the fiber optic core 34 by total internal reflection at the interface with clad 36. When sensor coating 32 is exposed to CO environment 38, coating 32 changes color, which attenuates the incident light beam 35 which is totally internally reflected from the region of core 34 covered by coating 32. An attenuated light beam 40 is output from sensor 30. Thus, sensor 30 is an ATR optical transducer, where the attenuation provides a measure of the CO environment. As shown in Fig. 2B, absorption sensor 42 is formed of a strip or block 44 which is made of the CO sensitive sensing chemistry in a suitable CO permeable solid state matrix. Strip or block 44 can be mounted on a suitable support 46. Incident light beam 48 passes straight through strip or block 44. Changes in color (absorption) caused by CO exposure produce an attenuated output beam 50. Instead of passing beam 48 through block/strip 44 parallel to support 46, the beam may pass through support 46 if the support 46 is transparent. Thus sensor 42 is an absorption type optical transducer, where transmitted beam attenuation provides a measure of the CO environment. In order to succeed commercially, the sensor has to be fast responding and reversible with a reproducible response. The sensor should be specific and it should have extended lifetime. Its production should be relatively easy. According to the invention, the following classes of materials are employed for building the solid state sensor: [A] Color forming agent: (l) Compounds of molybdenum, tungsten or vanadium, as above; (2) Any other organic compound or transition metal complex, eg. hemoglobin or its analogs, that show color change directly with CO or with reduced pelladium, ruthenium or osmium. [B] Catalyst: (1) Palladium (II) compounds as above; (2) Ruthenium (VIII) compounds as above; (3) Osmium (VIII) compounds as above; (4) Any other inorganic compound with multiple stable oxidation states having redox properties compatible to reduction by CO in the presence or absence of a modifier. [C] Reversing Agents: (1) Ferric(III) salts as above; (2) Chromium (VI) salts as above; (3) Cerium (IV) salts as above; (4) Any other organic compound or transition metal complex or inorganic compound capable of reverting the color reaction and itself being regenerable at ambient conditions. [D] Redox property modifier: (1) Salts of acetic acid, as above; or (2) Any other compound that prevents the color forming reaction in the absence of CO. [E] Interference suppressing agent: As above. This group includes ions forming a colorless or white precipitate with the interfering material or a precipitate which has a color which either does not overlap in the measurement window of wavelengths or has an overlap which is resolvable by applying smart computer software, for example, chemometrics. [F] Embedding Matrix: Film forming cross- linkable/polymeriz-able monomer and polymer, including but not limited to poly(vinyl chloride) (PVC) , carboxylated PVC, polystyrene, cellulose derivatives, variations of plexiglass, silanes, siloxaneε, silicones. The matrix could also be formed of gel-forming material, such as sol gel, silica gel, or hydrogel. The matrix should not have functionalities, e.g., OH (hydroxyl) , which reduce the color forming agent or catalyst. [G] Permeation enhancer: Plasticizer, including but not limited to tributyl phosphate, sebacic acid dibutyl ester, dioctyl phthalate. [H] Transducer support: Supporting structure for matrix with embedded chemistry, comprising but not limited to planar waveguide, optical fiber, slab, disc, prism, strips, rod, pipe, cube, film. The support materials include both amorphous and single-crystal or polycrystalline materials, inorganic and organic compounds. Types of glass include quartz, pyrex, sodalime, phosphate, borosilicate, fluoride, chalcogenide, fluorozirconate. The support can be integrated optic materials including but not limited to oxides, nitrides, sulfides, oxynitrides, zirconates, titanates. Polymers and organic materials can be pure or doped, including but not limited to plexiglass and polyimide. Single crystal materials include but are not limited to silicon, lithium niobate, lithium tantalate, barium tellurate, and garnets. Semiconductor materials can also be employed as a substrate. The sensing chemistry/matrix is deposited on the substrate by evaporation, lamination, spraying, dip-coating, casting and spreading, Q-tipping, spin-coating, etc. The support can be an operational part of the optical transducer as in Fig. 2A, or merely a support as in Fig. 2B. It can also take the form of containment means, as shown in Fig. 2C. Sensor 52 is formed of a gas permeable tubular membrane 54 filled with a jelled sensing chemistry matrix 56. Optical windows 58 are placed at the ends of tube 54 to pass input light beam 60 and output beam 62. The present invention is based on the judicial choice of one component from each category. In a preferred embodiment of the chemical sensor for carbon monoxide, molybdosilicic acid is the color forming agent; palladium sulfate is the catalyst; anhydrous ferric chloride is the reversing agent; sodium acetate is both the redox property modifier, and suppressor of hydrogen sulfide interference; PVC is the embedding matrix, while tributyl phosphate is the CO permeation enhancer. Finally, the sensing cocktail is coated on an optical fiber with a Q-tip. A typical formulation is described as follows: 1. Weigh 0.012 g Palladium sulfate. 2. Add 3 drops of freshly prepared 10% Na(OAc) 2 . 3. Mix thoroughly. 4. Add 0.048 g molybdosilicic acid and mix well. 5. Add 6 ml of 10% PVC in tetrahydrofuran, and mix thoroughly. 6. Add 24 drops of tributyl phosphate, and mix well. 7. Add 0.024 g of ferric chloride, and mix well. 8. Cast a portion of this batch of sensing chemistry/matrix on a glass slide for quality control; check using a uv/vis spectrophotometer. 9. Coat optical fibers with Q-tip. Figure 3 shows the performance (absorption spectra) of the sensing chemistry as determined by uv/vis. The chemistry is used on a glass slide, and the coating thickness is about 50 microns. Because of this very small path length, the glass slide is exposed to 100% CO. Curve 1 is the background before exposing the chemistry to 100% CO, Curve 2 is the spectrum after exposure of the chemistry to 100% CO for 30 minutes, Curve 3 is the spectrum taken after the sensing chemistry has reverted, and Curve 4 is the absorption spectrum taken after the chemistry was re-exposed to 100% CO. As Figure 3 shows, the interaction of CO with the chemistry produces a broad absorption spectrum. Figure 3 also shows the reproducibility of the sensor response. Figure 4 shows how quickly the chemistry reverts back to the starting stage. It is difficult to dissolve inorganic salts in an organic matrix. The small ion-counterion pairs, e.g., Pd +2 , S0 4 2 , behave as point charges and are expelled by the organic matrix. Thus it is necessary to improve the solubility of the inorganic salts which provide the color forming agent, catalyst and reversing agent in the polymer matrix to form a solid state CO sensor with fast response, high sensitivity and fast regeneration. According to the invention, the solubility of the inorganic salts in a polymer matrix is improved by counterion exchange with lipophilic counterions. The lipophilic counterions are counterions with hydrophobic chains which dissolve easily into organic media. Thus, the sulfate counterion can be exchanged with a pair of dodecylsulfate counterions, which contain a C 12 chain. The exchange process can be readily carried out. The palladium sulfate salt is placed in an aqueous solution. The dodecylsulfate surfactant is added to the solution. The palladium dodecylsulfate ion pair is extracted from the solution with non-polar organic solvents, and later recovered by evaporating the solvent. The palladium dodecylsulfate salt is then used to prepare the CO sensing chemistry. Similarly, lipophilic counterions can be added to the reversing agent or to the color forming agent. Figs. 6A-C show the Pd(II), Fe(III) and Ce(IV) ions with dodecylsulfate counterions. Fig. 6D shows a molybdenum oxide anion with hydrophobic quaternary ammonium cations, i.e., nitrogen with four long hydrophilic chains attached, e.g., groups R represent alkyl chains. The large organic counterions facilitate solubility of the ions in the polymer. A number of techniques can be applied for measuring CO. For example CO can be quantified by measuring the intensity of a band of wavelengths. The intensity modulation can arise from either attenuated total internal reflection phenomenon, or from a straight-through absorption process, as shown in Figures 2a, and 2b. Alternatively, as Figure 5 shows, carbon monoxide can be quantified from phase modulation or interferometric measurements. The interferometric sensor 64 has two arms, a sensing arm 66 containing sensing chemistry/matrix 68, and a reference arm 70 containing sensing chemistry/matrix 72. Sensing arm 66 is exposed to CO while reference arm 70 is not. An input light beam 74 having a well defined mode is split and input into arms 66,70. As sensing chemistry 68 reacts with CO, it changes the mode propagation characteristics of arm 66 so that the portion of light beam that traverses arm 66 will change its mode while the portion that traverses arm 70 will not. The outputs of arms 66, 70 are recombined to produce output beam 76. Because of the differences in modes caused by CO exposure, output beam 76 will exhibit an interference pattern 78. An interferometric sensor can also be implemented in a single waveguide channel by propagating a light beam having two modes, one of which is affected by the change in absorbance of the sensing chemistry on the waveguide. The change in interference pattern between the two modes is a measure of the CO exposure. Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.

Claims: CLAIMS 1. A CO sensor comprising, a CO sensing chemistry comprising: a color forming agent which is reduced by CO and produces a color change; a catalyst which is reduced by CO and which thereby also reduces the color forming agent; a reversing agent which oxidizes the reduced color forming agent back to its initial state; a redox property modifying agent which prevents reduction of the catalyst and the color forming agent in the absence of CO. The CO sensor of Claim 1 further comprising: an interference suppressing agent which preferentially forms noninterfering precipitates with interfering species to prevent the interfering species from forming precipitates with any of the color forming agent, catalyst, and reversing agent. The CO sensor of Claim 1 wherein the CO sensing chemistry is in solution. The CO sensor of Claim 1 wherein the redox property modifying agent is a source of acetate counterions. The CO sensor of Claim 4 wherein the source of acetate counterions is an acetic acid salt selected from the group consisting of sodium acetate, ammonium acetate, potassium acetate, lithium acetate, magnesium acetate and copper acetate. 6. The CO sensor of Claim 2 wherein the interference suppressing agent comprises a source of ions which form white or colorless precipitates with interfering species. The CO sensor of Claim 2 wherein the interference suppressing agent is a source of sodium, potassium, ammonium, calcium, lithium, magnesium or beryllium ions. 8. The CO sensor of Claim 1 wherein the color forming agent comprises a source of Mo +0 , W +0 or V +s ions; the catalyst comprises a source of Pd +2 , Ru +8 or Os +l< ions; the reversing agent comprises a source of Fe +3 , Cr +6 , or Ce +4 ions. 9. The CO sensor of Claim 1 further comprising: a polymer embedding matrix containing the CO sensing chemistry. 10. The CO sensor of Claim 9 wherein the CO sensing chemistry further comprises: an interference suppressing agent. 11. The CO sensor of Claim 9 further comprising a support on which the matrix with embedded chemistry is deposited. 12. The CO sensor of Claim 9 further comprising a light source for inputting a light beam into the matrix with embedded chemistry and a detector for measuring an attenuated light beam from the matrix with embedded chemistry. 13. The CO sensor of Claim 9 wherein the color forming agent comprises a source of Mo +0 , W +0 or V +5 ions; the catalyst comprises a source of Pd +2 , Ru +!i or Os +li ions; the reversing agent comprises a source of Fe +3 , Cr +ft , or Ce +4 ions. 14. The CO sensor of Claim 9 wherein the redox property modifier is an acetic acid salt. 15. The CO sensor of Claim 14 wherein the acetic acid salt is selected from the group consisting of sodium acetate, potassium acetate, magnesium acetate, copper acetate, ammonium acetate, lithium acetate. 16. The CO sensor of Claim 10 wherein the interference suppressing agent comprises a source of ions which form a colorless or white precipitate with an interfering species or a precipitate which has a color which does not overlap a measurement window of wavelengths or which has a resolvable overlap. 17. The CO sensor of Claim 16 wherein the interference suppressing agent is selected from the group consisting of sodium, potassium, calcium, ammonium, lithium, beryllium, magnesium, calcium salts with counterions selected from nitrate, acetate, chloride, sulfate, phosphate, chlorate, nitrite, carbonate, bicarbonate. 18. The CO sensor of Claim 9 wherein polymer is selected from the group consisting of poly(vinyl chloride) (PVC) , carboxylated PVC, polystyrene, cellulose derivatives, variations of plexi-glasε, silanes, siloxanes, silicones. 19. The CO sensor of Claim 11 wherein the support is an optical structure through which a light beam is transmitted to and from the matrix with embedded chemistry. 20. The CO sensor of Claim 19 wherein the optical structure is a planar or fiber optic waveguide. 21. The CO sensor of Claim 9 wherein any of the color forming agent, catalyst or reversing agent comprise an active ion with a lipophilic counterion. 22. In a solid state CO sensor having a color forming agent, a catalyst, and a reversing agent embedded in a porous matrix, the improvement comprising at least one of: selecting the catalyst from Ru +S or Os +!t ions; selecting the reversing agent from Cr +6 or Ce +4 ions. 23. A method for increasing the solubility of an inorganic salt having an ion and associated counterion in an organic matrix comprising exchanging the associated counterion with a lipophilic counterion and embedding the ion- lipophilic counterion pair in the organic matrix. 24. The method of Claim 23 wherein the lipophilic counterion includes a hydrophobic organic chain. 25. The solid state sensor of Claim 19 wherein the optical structure is a waveguide, and further comprising a source of an input light beam having two interfering modes, one of which is changed by changing absorption caused by a color change of the embedded chemistry. 26. The solid state sensor of Claim 19 wherein the optical structure comprises a Mach-Zehnder interferometer having a pair of arms, a sensing arm and a reference arm, each having the matrix with sensing chemistry formed thereon, wherein the sensing arm is exposed to a sample while the reference arm is not, wherein the mode of a light beam transmitted through the sensing arm is changed by changing absorption caused by a color change of the embedded chemistry and interferes with a light beam transmitted through the reference arm.

Inventors: EJIOFOR CHUKA H, GOSWANI KISHOLOY, KLAINER STANLEY M, SAINI DEVINDER P S, EJIOFOR, CHUKA, H., GOSWANI, KISHOLOY, KLAINER, STANLEY, M., SAINI, DEVINDER, P., S.
Applicants: FIBERCHEM INC, FCI-FIBERCHEM, INC.
Application year: 1994
Main classification: G01N21
IPC codes: G01N21, G01N31, G01N33